The present invention relates to ceramic cutting tool materials and particularly to such cutting tool materials with a ceramic matrix in which monocrystalline whiskers (hair crystals) of silicon carbide and small (significantly less than 1 .mu.m) particles of carbides, nitrides and/or borides of metals from group VB (V,Nb,Ta) are uniformly distributed resulting in an increased strength and toughness without negatively influencing the wear resistance of the material.
Ceramic cutting tool materials have been available for several decades. However, until recently they have not had any significant commercial importance for use in chipforming machining. The main reason for the limited growth of ceramic cutting tools has been sudden and unexpected tool failures because of their inherent inadequate strength and toughness.
In recent years, the properties of ceramic cutting tool materials have been improved in many respects and their use in cutting of cast iron and heat-resistant alloys (e.g., nickel-base alloys) has relatively increased.
Aluminum oxide based cutting tool materials are very sensitive to thermal crack formation because aluminum oxide in itself has a relatively poor thermal conductivity. This leads to very short tool lives in machining steel, particularly under conditions with short operating times and varying cutting depth.
To a certain extent, the thermal properties have been improved by particulate additions of titanium carbide and/or titanium nitride which enhance the thermal conductivity of the tool material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminum oxide materials, an increased tool life is therefore obtained in the cutting of harder work piece materials and in operations demanding thermal shock resistance. However, this kind of material has too poor a toughness behavior for a more general use in the cutting of steel.
A later development relates to alloying of uniformly dispersed fine-grained zirconium oxide particles in a matrix of aluminum oxide. A transformation of the `metastable` zirconium oxide particles during use increases both strength and toughness and thus leads to a more predictable tool life.
The thermal properties of said type of materials are, however, only slightly better than those of pure aluminum oxide materials. Therefore, initiation and growth of thermally induced cracks is still a great problem in practical cutting operations generating high cutting edge temperatures such as cutting of steel.
It has recently been shown (T. N. Tiegs and P. F. Becher, J. Am. Ceram. Soc. 90(5) C-109-C-111, 1987) that alloying of SiC-whiskers, with mono-crystalline hair crystals, in a matrix of aluminum oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting tool materials based upon said concept have shown very good performance in the cutting of heat-resistant materials in particular.
It is well-known that particulate additions can be used to improve the properties of a brittle ceramic material, e.g., U.S. Pat. No. 4,320,203 which relates to additions of TiN and Ti(C,N).
Depending on the nature of the additions the operating toughening mechanisms can be crack deflection, microcracking, transformation toughening or crack bridging. It is characteristic for these particulate additions that the size of the particles are of the same order of magnitude as the matrix material e.g., in the order of 1-5 .mu.m and that they are located in the grain boundaries of the matrix material. Only in the case of ZrO.sub.2 -additions is a smaller grain size than the matrix needed in order to suppress a transformation during fabrication of the material. However, the zirconia particles are still predominantly located in the grain boundaries of the alumina matrix material.
It is also characteristic for the particulate reinforced materials that property improvements are achieved up to rather high particulate contents, normally up to 15-30 percent by volume.
It has recently been found that further improvements of the properties are possible in the silicon carbide whisker-reinforced cutting tool materials, especially the strength, if small additions of nanosize particles are added to the whisker reinforced materials. In U.S. Pat. No. 5,123,935 such a material is described containing nanosize Ti-compounds. It is believed that the function of the small Ti-compounds is to strengthen the alumina matrix and to refine the microstructure.